Thiazole-induced rigidification in substituted dithieno-tetrathiafulvalene: the effect of planarisation on charge transport properties

Two novel tetrathiafulvalene (TTF) containing compounds 1 and 2 have been synthesised via a four-fold Stille coupling between a tetrabromo-dithienoTTF 5 and stannylated thiophene 6 or thiazole 4. The optical and electrochemical properties of compounds 1 and 2 have been measured by UV–vis spectroscopy and cyclic voltammetry and the results compared with density functional theory (DFT) calculations to confirm the observed properties. Organic field effect transistor (OFET) devices fabricated from 1 and 2 demonstrated that the substitution of thiophene units for thiazoles was found to increase the observed charge transport, which is attributed to induced planarity through S–N interactions of adjacent thiazole nitrogen atoms and TTF sulfur atoms and better packing in the bulk.

Full experimental and characterisation for compounds 1, 2 and 4-6, as well as OFET fabrication methods and AFM images of OFETs containing 2.

General remarks
Pd(PPh 3 ) 4 was prepared prior to use and stored in a freezer under nitrogen. Unless otherwise stated, all other reagents were sourced commercially and used without further purification. 1 H and 13 C NMR spectra were recorded on a Bruker Avance DPX400 apparatus at 400.1 and 100.6 MHz respectively or on a Bruker Avance III at 600.1 and 150.9 MHz respectively. Chemical shifts are given in ppm, all J values are quoted in Hz. Low resolution mass spectrometry (LRMS) was performed on a Shimadzu Axima-CFR spectrometer (MALDI). High resolution mass spectrometry (HRMS) was gratefully S2 performed by the EPSRC national facility at Swansea. Dry solvents were obtained from a solvent purification system (SPS 400 from Innovative Technologies). Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer Thermogravimetric Analyzer TGA7 under a constant flow of argon.
Melting points were recorded using a TA instruments DSC QC1000 Differential Scanning Calorimeter and are uncorrected.
Cyclic voltammetry (CV) measurements were performed on a CH Instruments 660A electrochemical workstation with iR compensation using anhydrous dichloromethane as the solvent and tetrabutylammonium hexafluorophosphate as the electrolyte at 0.1 M. The electrodes used were glassy carbon, platinum wire and silver wire as the working, counter and reference electrodes respectively. All solutions were degassed using Ar and contained the analyte compound at concentrations of ca. 10 −4 M. All measurements were referenced against the E 1/2 of the Fc/Fc + redox couple and HOMO/LUMO levels were calculated assuming a ferrocene HOMO level of −4.8 eV.
Absorption spectra were recorded using a Shimadzu UV 7200 instrument.
Organic field-effect transistors were fabricated on SiO 2 substrates with prefabricated interdigitated Au source-drain channels with lengths of 2.5, 5, 10 and 20 µm and width of 1 cm. N-doped Si and SiO 2 were the gate electrode and gate dielectric, respectively. The substrates were cleaned using water, acetone and ethanol before being treated in UV-ozone for 30 seconds. The pentafluorobenzenethiol (PFBT) self-assembled monolayer (SAM) was prepared by dropcasting a solution of PFBT (10 mM in ethanol) onto the substrate. After 1 min, the residual PFBT was then washed with ethanol and the substrate was dried over a stream of compressed air. Similarly, octadecyltrichlorosilane (OTS) SAM was prepared by dropcasting an OTS solution (13 mM in toluene) onto the substrate which was washed with toluene and dried after 1 min. Devices utilising both SAMs were prepared by sequential dropcasting of the SAM solutions as above (PFBT then OTS). Compounds 4 and 5 were deposited via spin-coating 10 mg ml -1 chloroform and o-dichlorobenzene solutions. Films of compounds 4 and 5 were annealed at 100 °C and 120 °C respectively. Current-voltage characteristics were recorded using a Keithley 4200 semiconductor parameter analyser at room temperature in a nitrogen atmosphere. The field-effect mobilities were determined from the saturation regime and calculated using the following equation: The surface morphology was characterised using a Dimension 3100 atomic force microscope (AFM) in tapping mode.

2-Bromo-4-hexylthiazole
1-Thiocyanatooctan-2-one (4.00 g, 16.1 mmol) was added to a 33 % solution of HBr in acetic acid (45 ml) and stirred overnight at room temperature. The reaction mixture was then poured onto ice and neutralised carefully with NaOH keeping the temperature below 10 °C. The mixture was extracted with CH 2 Cl 2 (3 × 50 ml) and the combined extracts washed with brine (100 ml) and dried over Na 2 SO 4 . The crude product was purified by silica gel column (hexane:CH 2 Cl 2 , 9:1) to give 2-bromo-